<!--
 * @Description: 歌手
 * @version:
 * @Author: GanEhank
 * @Date: 2019-08-04 02:31:14
 * @LastEditors: GanEhank
 * @LastEditTime: 2019-08-14 16:39:56
 -->
<template>
  <scroll
    class="listview"
    :data="data"
    ref="listview"
    :listenScroll="listenScroll"
    @scroll="scroll"
    :probeType="3"
  >
    <ul>
      <li v-for="(group, index) in data" :key="index" class="list-group" ref="listGroup">
        <h2 class="list-group-title">{{group.title}}</h2>
        <ul>
          <li
            v-for="(item, index) in group.items"
            :key="index"
            class="list-group-item"
            @click="selectItem(item)"
          >
            <img v-lazy="item.avatar" class="avatar" />
            <span class="name">{{item.name}}</span>
          </li>
        </ul>
      </li>
    </ul>
    <div
      class="list-shortcut"
      @touchstart.stop.prevent="onShortcutTouchStart"
      @touchmove.stop.prevent="onShortcutTouchMove"
    >
      <ul>
        <li
          v-for="(item, index) in shortcutList"
          :key="index"
          class="item"
          :data-index="index"
          :class="{'current': currentIndex === index}"
        >{{item}}</li>
      </ul>
    </div>
    <div class="list-fixed" v-show="fixedTitle" ref="fixedTitle">
      <div class="fixed-title">{{fixedTitle}}</div>
    </div>
    <div class="loading-container" v-show="!data.length">
      <loading></loading>
    </div>
  </scroll>
</template>

<script>
import { getData } from '@/common/js/dom'
import Scroll from '@/base/scroll/scroll'
import Loading from '@/base/loading/loading'

const ANCHOR_HEIGHT = 18 // 通过样式设置计算得到
const TITLE_HEIGHT = 30

export default {
  components: {
    Scroll,
    Loading
  },
  props: {
    data: {
      type: Array,
      default: null
    }
  },
  data() {
    return {
      scrollY: -1,
      currentIndex: 0, // 当前显示的第几个title项
      diff: -1 // fixed title的偏移位置
    }
  },
  computed: {
    shortcutList() {
      return this.data.map(group => { // 得到title的集合数组，‘热门’取1个字
        return group.title.substr(0, 1)
      })
    },
    fixedTitle() {
      if (this.scrollY > 0) {
        return ''
      }
      return this.data[this.currentIndex]
        ? this.data[this.currentIndex].title
        : ''
    }
  },
  created() {
    this.touch = {}
    this.listenScroll = true
    this.listHeight = []
  },
  methods: {
    selectItem(item) {
      this.$emit('select', item)
    },
    onShortcutTouchStart(e) {
      const anchorIndex = getData(e.target, 'index') // 获取data-index的值  index 得到的是字符串
      const firstTouch = e.touches[0]
      this.touch.y1 = firstTouch.pageY
      this.touch.anchorIndex = anchorIndex
      this._scrollTo(anchorIndex)
    },
    onShortcutTouchMove(e) {
      const firstTouch = e.touches[0]
      this.touch.y2 = firstTouch.pageY
      const delta = ((this.touch.y2 - this.touch.y1) / ANCHOR_HEIGHT) | 0 // 获取列表项差值，| 0 向下取整
      const anchorIndex = parseInt(this.touch.anchorIndex) + delta
      this._scrollTo(anchorIndex)
    },
    refresh() {
      this.$refs.listview.refresh()
    },
    scroll(pos) {
      this.scrollY = pos.y // 实时获取BScroll滚动的Y轴距离
    },
    _scrollTo(index) {
      if (!index && index !== 0) {
        return
      }
      if (index < 0) {
        index = 0
      } else if (index > this.listHeight.length - 2) {
        index = this.listHeight.length - 2
      }
      this.scrollY = -this.listHeight[index] // 点击时，手动得到元素上限height位置，改变scrollY, 使点击项高亮
      this.$refs.listview.scrollToElement(this.$refs.listGroup[index], 0) // 列表滚动定位 第二个参数表示：要不要滚动动画缓动时间; 0 瞬间滚动
    },
    _calculateHeight() {
      this.listHight = [] // 每次重新计算每个group高度时，恢复初始值
      const list = this.$refs.listGroup
      let height = 0 // 初始位置的height为0
      this.listHeight.push(height)
      for (let i = 0; i < list.length; i++) {
        const item = list[i] // 得到每一个group的元素
        height += item.clientHeight // DOM元素可以用clientHeight获取元素高度
        this.listHeight.push(height) // 得到每一个元素对应的height
      }
    }
  },
  watch: {
    data() {
      setTimeout(() => {
        this._calculateHeight()
      }, 20)
    },
    scrollY(newY) {
      const listHeight = this.listHeight
      if (newY > 0) { // 当滚动到顶部，newY>0
        this.currentIndex = 0
        return
      }
      for (let i = 0; i < listHeight.length - 1; i++) { // 在中间部分滚动，遍历到最后一个元素，保证一定有下限，listHeight中的height比元素多一个
        const height1 = listHeight[i]
        const height2 = listHeight[i + 1]
        if (-newY >= height1 && -newY < height2) {
          this.currentIndex = i
          this.diff = height2 + newY // 得到fixed title上边界距顶部的偏移距离
          return
        }
      }
      // 当滚动到底部，且-newY大于最后一个元素的上限
      // currentIndex 比listHeight中的height多一个, 比元素多2个
      this.currentIndex = listHeight.length - 2
    },
    diff(newVal) {
      const fixedTop =
        newVal > 0 && newVal < TITLE_HEIGHT ? newVal - TITLE_HEIGHT : 0
      if (this.fixedTop === fixedTop) {
        return
      }
      this.fixedTop = fixedTop
      this.$refs.fixedTitle.style.transform = `translate3d(0, ${fixedTop}px, 0)`
    }
  }
}
</script>

<style lang="stylus" scoped>
@import '../../common/stylus/variable';

.listview {
  position: relative;
  width: 100%;
  height: 100%;
  overflow: hidden;
  background: $color-background;

  .list-group {
    padding-bottom: 30px;

    .list-group-title {
      height: 30px;
      line-height: 30px;
      padding-left: 20px;
      font-size: $font-size-small;
      color: $color-text-l;
      background: $color-highlight-background;
    }

    .list-group-item {
      display: flex;
      align-items: center;
      padding: 20px 0 0 30px;

      .avatar {
        width: 50px;
        height: 50px;
        border-radius: 50%;
      }

      .name {
        margin-left: 20px;
        color: $color-text-l;
        font-size: $font-size-medium;
      }
    }
  }

  .list-shortcut {
    position: absolute; // 绝对定位到右侧
    right: 0;
    top: 50%;
    z-index: 30;
    transform: translateY(-50%);
    width: 20px;
    padding: 20px 0;
    border-radius: 10px;
    text-align: center;
    background: $color-background-d;
    font-family: Helvetica;

    .item {
      padding: 3px;
      line-height: 1;
      color: $color-text-l;
      font-size: $font-size-small;

      &.current {
        color: $color-theme;
      }
    }
  }

  .list-fixed {
    position: absolute; // 绝对定位到顶部
    top: 0;
    left: 0;
    width: 100%;

    .fixed-title {
      height: 30px;
      line-height: 30px;
      padding-left: 20px;
      font-size: $font-size-small;
      color: $color-text-l;
      background: $color-highlight-background;
    }
  }

  .loading-container {
    position: absolute;
    width: 100%;
    top: 50%;
    transform: translateY(-50%);
  }
}
</style>
